Conventionally, in an image forming device such as an electrophotographic copy machine, prior to forming an electrostatic latent image at a body to be charged such as a photoreceptor, the surface of the body to be charged is evenly charged by a variety of methods.
Conventional charging methods include, for example, corona discharge. In this method, the surface of a photoreceptor is charged by discharge from an extremely fine wire. This method, however, has a problem in that a high voltage power supply of about 4 to 10 kV is necessary to charge the surface of a photoreceptor. Moreover, the discharge from the wire causes a large amount of ozone in the space between the wire and the surface of the photoreceptor, which adversely effects human bodies and also accelerates degradation of the photoreceptor. In order to solve the problem, a corona charger that is improved to decrease the resulting amount of ozone is disclosed, for example, in Japanese Patent Laying-Open Nos. 09-114192 and 06-324556.
Another charging method employs a contact charging scheme, which has recently been put into practical use. In this method, in order to decrease the resulting amount of ozone and the power consumption, a conductive member such as a conductive roller, brush, elastic blade or carbon nanotube is brought into contact with a surface of a photoreceptor to charge the surface of the photoreceptor.
Currently, a roller charging scheme using a conductive roller as a conductive member is widely utilized in light of the stability of charging. In the roller charging scheme, a conductive roller is abutted under pressure on a photoreceptor by receiving a voltage to attain the charging of the photoreceptor. In the roller charging scheme, however, when the surface of the photoreceptor has any minute defect (pinhole), an abnormal amount of current leak occurs in the defect portion of the surface of the photoreceptor from the conductive roller, causing the surface of the photoreceptor to be destroyed, which may adversely affect image formation.
As a further improvement to the roller charging scheme, for example, Japanese Patent Laying-Open No. 2001-296722 discloses a scheme in which a secondary charging roller is added between a roller charging member (primary charging roller) and a photoreceptor. Here, the secondary charging roller serves to carry charges from the primary charging roller to the photoreceptor and aims to resolve the current leak problem caused by the pinhole in the photoreceptor. Also in this scheme, however, the charging phenomenon is dominated by minute discharge created in the narrow gap between the secondary charging roller and the photoreceptor. Therefore, it was impossible to completely remove ozone or NOx produced during charging.
Furthermore, for example, Japanese Patent Laying-Open No. 2001-281964 discloses that a carbon nanotube is applied to a contact-type charger. In the contact-type charger using a carbon nanotube, however, the pressing pressure of the carbon nanotube in contact with a photoreceptor causes physical destruction of the carbon nanotube and accordingly reduces the charging ability.
In addition, Japanese Patent Laying-Open No. 2001-331017 discloses a charger using an electron emitting element having an MIS (Metal-Insulator-Semiconductor) structure. In the electron emitting element, a thin film electrode for forming an acceleration electric field for electrons is provided on the front surface side of a porous semiconductor layer, and an electrode for injecting electrons to the porous semiconductor layer is provided on the back surface side of the porous semiconductor layer. The electron emission principle and fabrication method for the porous semiconductor layer formed of a porous silicon thin film is disclosed in detail in “Luminescence and Related Novel Functions of Quantum-sized Nanosilicon”, the Technical Report of the Institute of Electronics Information and Communication Engineer of Japan, 1999-06, pp. 1-6. The charger using such an element only utilizes electron attachment caused by electrons emitted from the electron emitting element in order to generate negative ions and therefore does not produce ozone or NOx in principle as in the method using discharge as described above.
In the electron emitting element using the porous semiconductor layer, however, an electron, which attaches to a nanosized semiconductor particle (nano-silicon crystal) constituting the porous semiconductor layer due to charging (electron capture) caused during the operation of electron emission into the atmosphere, renders the electric field inside the porous semiconductor uneven, thereby preventing acceleration of electrons. The amount of electron emission is thus reduced. The electron stored in the nanosized semiconductor particle by this charging exhibits nonvolatility, and it is reported that some experimental result shows that the electron attached to a nanosized semiconductor particle over a week or longer. Generally, when this element is driven in the atmosphere, this charging causes electron emission from the electron emitting element to stop completely with continuous driving for about three minutes.